Sizing Up Titans—Navajo Erg vs. Sahara Ergs; Which was the larger sand box?

Some 185 million years ago, during the Early Jurassic, an enormous “sea” of dune fields called the Navajo erg covered most of eastern and southern Utah as well as parts of Idaho, Wyoming, Colorado, New Mexico, Arizona, Nevada, and California. This vast and ancient sand sea is now exposed as the Navajo, Nugget, Aztec, and Glen Canyon Sandstones (the name varies with location). These formations tend to form colorful and massive cliff faces that play leading, supporting, or cameo roles in the spectacular scenery at the following parks:

• Red Rock Canyon National Conservation Area
• Valley of Fire State Park

Wyoming

• Grand Teton National Park

The Navajo erg, with its resultant rock formations, is immensely impressive! But how does its vastness compare to modern analogs?

Multiple authors, myself included, have described it as “bigger than the dune fields of the modern Sahara” (see the May 2012 issue of Survey Notes) or some variant of that claim. I recently had an inquiry questioning the validity of the claim. Was the Navajo erg bigger than the dune fields of the modern Sahara or is that claim just oft repeated dogma?

The answer is dependent upon the specific phrasing of the claim and the extent of the ancient Navajo erg versus its modern rock remnant.

First consider the modern Sahara. The Navajo erg was not bigger than the entirety of the modern Sahara Desert. Estimates vary but the Sahara Desert is roughly 3.3 million square miles. By comparison, the contiguous United States is 3.1 million square miles.

However, like the modern deserts of Utah, the modern Sahara is composed of many environments in addition to sand dunes. Contrary to Hollywood portrayals of one endless sand sea, the Sahara has several ergs that are isolated by vast expanses of dry valleys (wadis), gravel plains (regs), rocky plateaus (hamadas), salt pans (chotts), and mountains (tassilis). Again, estimates vary but dune fields only cover 15 to 20% of the Sahara Desert, which equals roughly 495,000 to 660,000 square miles.

Size estimates for individual ergs of the Sahara are difficult to find but perhaps the biggest Sahara erg is the Grand Erg Oriental which covers about 119,000 square miles. Note that this estimate of the Grand Erg Oriental includes small non-dune areas within the erg. Of these estimated 119,000 square miles, roughly 70% is sand-covered (for more details see U.S. Geological Survey Professional Paper 1052, A Study of Global Sand Seas, 1979).

So, was the Navajo erg bigger than the individual dune fields or the combined dune fields of the Sahara? This brings us to the second consideration—how big was the Navajo erg?

Though outcrops of Navajo, Nugget, Aztec, and the Glen Canyon Sandstones are found over a vast area of some 230,000 square miles, they certainly do not show the full extent of the Navajo erg. Much of the original erg was removed by erosion. Many maps and figures that depict the extent of the Navajo erg refer to a 1983 paper by geologists Kocurek and Dott (see Jurassic Paleogeography and Paleoclimate of the Central and Southern Rocky Mountain Region in Symposium on Mesozoic Paleogeography of West-Central U.S.: Society for Sedimentary Geology, Rocky Mountain Section).

Kocurek and Dott approximate the full extent of the Navajo erg by using various lines of evidence, such as nearby equivalent age rock formations that contain sand dune deposits (eolian sandstones) inter-bedded with non-dune deposits. Furthermore, they suggest that at its full extent the erg included only local sand-free regions, notably at the Ancestral Rockies in central Colorado and Mogollon Highlands in central Arizona.

The maximum area of the Navajo erg as depicted by Kocurek and Dott covers approximately 850,000 square miles. Thus, the Navajo erg is likely to have been larger than the combined dune fields of the modern Sahara.

Side Note: The dune fields of the modern Sahara are compared to the Navajo erg because they are the most famous modern dune fields. However, they are not the largest modern dune fields. That distinction goes to the Rub’ al Khali erg, which covers between 200,000 and 300,000 square miles, most of the southern Arabian Peninsula.

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